docs: def call(self, params, _context, **kwargs):
          '''Note that we're returning a Promise object here, and bypassing the
          helper functionality that normally sets the results struct from the
          returned object. Instead, we set _context.results directly inside of
          another promise'''
  
          assert len(params) == self.paramCount
          # using setattr because '=' is not allowed inside of lambdas
          return evaluate_impl(self.body, params).then(lambda value: setattr(_context.results, 'value', value))